Trainable performance upper bounds for image and video captioning

The task of associating images and videos with a natural language description has attracted a great amount of attention recently. Rapid progress has been made in terms of both developing novel algorithms and releasing new datasets. Indeed, the state-of-the-art results on some of the standard datasets have been pushed into the regime where it has become more and more difficult to make significant improvement. This work takes a step back, and begs two questions: "what is the best performance one could possibly achieve on a specific dataset?" and "How many visual elements does a given model capture?". To answer the first question, we first utilize existing natural language parsers to extract key concepts from ground truth captions. Then a conditional language model is trained to reproduce the original captions given various amount of extracted conceptual hints. By adjusting the amount of visual hints to the language model, we establish empirically dataset-dependent upper bounds on various automatic evaluation metrics commonly used to compare models. We demonstrate the construction of such bounds on MS-COCO, YouTube2Text and LSMDC (a combination of M-VAD and MPII-MD). As an upper bound, it suggests the best possible performance one could achieve on a particular dataset. To answer the second question, the current state-of-the-art results are compared against the proposed upper bounds. Based on such a comparison, we experimentally quantify several important factors concerning image and video captioning: the number of visual concepts captured by different models, the trade-off between the amount of visual elements captured and their accuracy,and the intrinsic difficulty and blessing of different datasets.